Because the cerebral endothelium is so completely ensheathed by astrocytes, it is possible that the two cell types interact so as to modulate the structure of their plasma membranes. The capillary endothelial cells have complex, zonular tight junctions (TJ), and the perivascular astrocytic membranes have localized high concentrations of particle assemblies (300-500/um2). When the endothelial cells were separately grown as enriched subcultures, the constituent strands of the endothelial TJ, viewed in freeze fracture, were reduced to a few, isolated patches. Among the strands lay many gap junctions which, in situ, are known to be absent from mature, brain capillaries. When the two cell types were co-cultured, the endothelium had greatly enhanced TJ: broad arrays of long, continuous, interconnecting strands. The distributions of both TJ length and width between solo endothelial cultures and co-cultures were significantly different. The area occupied by gap junctions was markedly reduced from a mean of 27% in solo, endothelial cultures, to 8% in co-cultures. The endothelial junctions were thus "normalized" by the astroglia. Substitution of astroglia with fibroblasts or aortic smooth muscle cells had no effect on the TJ. The astrocytes, in solo culture had randomly distributed, low numbers of assemblies (1/um2), which, in co-culture became aggregated while the assembly density of the non-aggregated areas increased 5 fold. The assemblies of astrocytes, grown on purified laminin or with fibroblasts instead of endothelium, were unaltered. Thus, in vitro and, perhaps in vivo, the astroglia may modulate endothelial junctions involved in the blood-brain barrier, while the endothelium affects the number and distribution of assemblies within the astrocyte cell membrane.